9. Jupiter: a giant primitive planet

Beneath Jupiterís clouds

Educated guesses about Jupiterís internal constitution

We cannot see beneath the clouds of Jupiter, but we can use external measurements to constrain its internal properties. As an example, we now know that the giant planet emits its own heat radiation, which means that it is hot inside. Since Jupiter and the Sun originated from similar material at the same time, a good initial assumption is that they have the same ingredients with similar proportions. The planetís low average mass density indicates that it is in fact composed largely of hydrogen and helium, just as the Sun is. The planetís oblate shape and rapid rotation also tell us something about the way it is constructed inside. Due to the enormous pressures inside Jupiter, most of the planetís hydrogen is compressed into a liquid metallic form, which has been created in the terrestrial laboratory and helps account for the giantís strong magnetic field. All of these constraints have been pieced together to make a picture of Jupiterís invisible interior.

An incandescent globe

With the advent of ground-based infrared measurements of the planets, pioneered by Frank J. Low and his colleagues in the 1960s, astronomers were surprised to discover in 1969 that the giant planet is an incandescent globe with its own internal source of heat. This result was confirmed in greater detail with instruments aboard the Voyager 1 and 2 spacecraft, that determined precisely how much infrared heat radiation was emerging from inside the planet. They showed that Jupiter is radiating 1.67 times as much energy as the atmosphere absorbs from incoming sunlight. In other words, the giant planet radiates nearly twice as much energy as it receives from the Sun, and almost half of the total energy that Jupiter loses must come from its interior. That essentially meant that the planet had to be unexpectedly hot inside.

Ingredients at formation

According to the widely accepted nebular hypothesis, the Sun and planets formed together during the collapse of a rotating interstellar cloud called the solar nebula. Most of it fell into the center, until it became hot enough to ignite the Sunís nuclear fires. Further out, the planets formed out of a whirling disk of the same material. If the nebular hypothesis is correct, and the whole solar system originated at the same time, then you might expect Jupiter to have a similar chemical composition to the Sun. To a first approximation, the abundance of the elements in the giant planet does indeed mimic that of the Sun, with a predominance of the lightest element hydrogen. It is the most abundant element in most stars, in interstellar space, and in the entire Universe. The second most abundant element in both Jupiter and the Sun is helium, and hydrogen and helium together account for the low mean mass density of both objects, at 1,330 and 1,409 kilograms per cubic meter respectively.

An equatorial bulge

Observations with even a small telescope show that Jupiter is not a sphere. It has a perceptible bulge around its equatorial middle and is flattened at the poles. This elongated oblate shape is caused by Jupiterís rapid spin. The outward force of rotation opposes the inward gravitational force, and this reduces the pull of gravity in the direction of spin. Since this effect is most pronounced at the equator, and least at the poles, the planet expands into an oblate shape that is elongated along the equator. The same thing happens to all the giant planets, and even to the solid Earth.

The amount of Jupiterís non-spherical extension depends on both its rate of rotation and the internal distribution of its material. The faster the spin, the more the outward push and the greater the elongation. And given its rotation, with the rapid period of 9.9249 hours, the size of the equatorial bulge depends on how Jupiterís mass is distributed inside. The more massive the planetís dense core, the smaller the equatorial bulge.

If Jupiter were a perfectly spherical planet, it would act as if all its mass was concentrated in a single central point and the motions of natural satellites or spacecraft would not depend on their orientation with respect to the planetís equator. In contrast, an oblate planet produces an extra force that tugs the moving object toward its equatorial bulge, and also toward any internal core. When combined with the known mass, volume and rotation rate of Jupiter, observations of these effects indicate that Jupiter has a dense core containing up to 12 Earth masses. Such a central object, presumably composed of non-gaseous rock and ice, was apparently required to initiate the accumulation of the giant planetís extensive hydrogen shell, which now compresses the core to high temperatures and pressures

Enormous pressures and strange matter

To understand the internal constitution of Jupiter, we need to know what happens to its most abundant ingredient, hydrogen, as the pressure increases. At the low pressure in the outer, visible parts of Jupiter, hydrogen forms a molecular gas, but this atmosphere is just a thin veneer. In proportion, the outer layer of gaseous hydrogen molecules resembles the skin of an apple. Deeper down, where the pressures and temperatures are higher, the hydrogen is liquefied. Indeed the planet is almost entirely liquid. It is mostly just a vast, global sea of liquid hydrogen.

Most of Jupiter is in the form of liquid metallic hydrogen. And underneath it all is a relatively small core of molten rock and ice. This relatively little core is up to twelve times heavier than the Earth. Theoretical considerations suggest that Jupiterís hydrogen probably accumulated around such a massive, pre-existing central object.

Electrical currents, driven by Jupiterís fast rotation within its liquid metallic shell, apparently generate the planetís strong magnetic field, in much the same way that electricity in the Earthís molten metallic core produces our planetís magnetism. Jupiterís magnetic field is much more powerful than Earthís magnetism, with a magnetic moment that is 20,000 times as large and a cloud-top strength that is about 14 times Earthís surface magnetic field strength. The greater strength of Jupiterís magnetism could be attributed to the planetís faster rotation, more extensive metallic region, and the relative proximity of the internal electrical currents to the cloud tops. By way of comparison, Earthís magnetic field is produced within a much smaller metallic core, which extends only half way to the surface.